Mice of the BALB/c strain injected with a line of the syngeneic T cell lymphoma S-49 which grows in suspension accept the tumor and die within two weeks. BALB/c mice injected with a plastic adherent (7.3) line of the same tumor are not killed. Furthermore, mice injected first with the 7.3 line and subsequently challenged with TAS are protected from the syngeneic tumor. Spleen cells from mice protected with 7.3 are challenged with TAS can be adoptively transferred to naive BALB/c mice which then protects these recipients. The 7.3 cell line appears to produce a factor in culture that stimulates growth of B lymphocytes. This may be a factor involved in immune protection against the metastatic TAS line.